It is known to transmit signals as optical radiation. The following prior art methods can be distinguished.
(a) remote operative systems PA1 The transmission of information is typically accomplished by directly varying the optical output of a radiation source with a digital information signal. For example, a remote control transmitter used for entertainment electronics operates in this manner. This system has the disadvantage that only one information signal can be transmitted at a time in a single direction. PA1 (b) sound transmission systems PA1 The infrared transmission of an audio signal, e.g., the transmission for the television accompanying sound to a battery-operated head receiver, is known. Such systems modulate the frequency of a subcarrier signal with an information base band signal. The modulated signal varies the output of an infrared radiation source with the momentary frequency of the modulated subcarrier. The system is disadvantageous because a direct visual contact between the optical emitter and the optical receiver is necessary. Further, information is only transmitted in one direction. PA1 (c) bidirectional sound transmission system PA1 An infrared sound transmission according to the above example (b) is known which supports bidirectional transmission. In establishing a bidirectional connection in such systems, each transceiver station must be designed such that the modulated radiation emitted by the optical emitter of each station impinges only on the receiver of the other station, i.e., an emitted signal from one station should not impinge on the detector of the same station. See W. Hirschmann, "Lichttelefon," Elektor, May, 1983, p. 5-53 to 5-52;; J. Stekelenburg, "Weitbereichs-IR-Sender/Empfanger," Elektor, November, 1987, p. 52-57. This system has the disadvantage that the stations communicating with one another have to be exactly aligned with respect to one another.
Furthermore, it is possible to transmit electrical signals as frequency modulated optical signals between two devices by providing each device with a transmitter and a receiver, wherein each transmitter and receiver includes a frequency oscillator for frequency modulation or frequency demodulation. Accordingly, a total of four oscillators and four different frequencies are required. Such systems, therefore, have a high cost.
German disclosure document 38 32 48b discloses a data processing system including a central processor and a plurality of data processing terminals located at least several meters from the central processor. The processor and each of the data processing terminals are provided with emitters and receivers for transmitting and receiving infrared signals. The transmission of data is realized by means of infrared radiation wherein the processor communicates with each data processing terminal successively. Accordingly, such an arrangement allows the bidirectional exchange of information between the central unit and periphery. However, this system does not support simultaneous bidirectional exchange of information.